In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. In a basic structure of such a light-emitting element, a layer containing a substance having a light-emitting property is interposed between a pair of electrodes. By application of voltage to this element, light emission from the substance having a light-emitting property can be obtained.
Since such a light-emitting element is of self-luminous type, it has advantages over a liquid crystal display, such as high visibility of the pixels and no need of a backlight, and is considered to be suitable as a flat panel display element. In addition, such a light-emitting element can be manufactured to be thin and light, which is a great advantage. Moreover, such a light-emitting element also has a feature that response speed is extremely fast.
Furthermore, since such a light-emitting element can be formed into a film form, planar light emission can be easily obtained by formation of a large-area element. This cannot be easily achieved with a point light source typified by an incandescent lamp or an LED, or with a line light source typified by a fluorescent lamp. Therefore, the light-emitting element has a high utility value as a plane light source which can be applied to lighting and the like.
The light-emitting elements utilizing electroluminescence are roughly classified in accordance with whether a substance having a light-emitting property is an organic compound or an inorganic compound. In the present invention, an organic compound is used as the substance having a light-emitting property. In that case, by application of voltage to the light-emitting element, electrons and holes are injected from a pair of electrodes into a layer containing an organic compound having a light-emitting property, so that current flows. Then, by recombination of these carriers (electrons and holes), the organic compound having a light-emitting property forms an excited state, and light is emitted when the excited state returns to a ground state.
Based on this mechanism, such a light-emitting element is referred to as a current excitation type light-emitting element. Note that there can be a singlet excited state and a triplet excited state as the type of the excited state formed by the organic compound, and light emitted in the case of the singlet excited state is referred to as fluorescence, and light emitted in the case of the triplet excited state is referred to as phosphorescence.
There are many problems which depend on substances in improving element characteristics of such a light-emitting element. Therefore, improvement of an element structure, development of a substance, and the like have been carried out in order to solve the problems.
For example, in Non-Patent Document 1 (: Tetsuo TSUTSUI, and eight others, Japanese Journal of Applied Physics, Vol. 38, L1502-L1504 (1999)), by provision of a hole-blocking layer, a light-emitting element using a phosphorescent substance efficiently emits light. As described in Non-Patent Document 1, however, there are problems in that the hole-blocking layer does not have durability and the lifetime of the light-emitting element is extremely short.